Carbon nanotubes have evoked considerable interest since their discovery in the early 1990s. Potential uses include everything from transistors, digital memory, and miniature electron emitters for displays, to hydrogen gas storage devices for the next generation of environmentally-friendly automobiles.
With mechanical strengths up to 100 times that of steel, carbon nanotubes are structurally seamless cylindrical tubes of graphite sheets. The basic repeating unit of the graphite sheet comprises hexagonal rings of carbon atoms, with a carbon-carbon bond length of about 1.42 Å. Carbon nanotubes may be capped with a fullerene hemisphere, and, depending on the method of synthesis, may comprise multi-walled or single-walled tubes. A single-walled carbon nanotube comprises a tube, the wall of which is only a single atom thick. Multi-walled carbon nanotubes comprise a collection of tubes, stuffed one within another in a nested configuration. A typical single-walled carbon nanotube (“SWNT”) may have a diameter varying from about 1 nanometer (“run”) to about 5 nm, and a length of up to a few millimeters. Generally, SWNTs are preferred over multi-walled carbon nanotubes for use in applications because they have fewer defects and are therefore stronger and more conductive than multi-walled carbon nanotubes having similar dimensions.
The structural characteristics of carbon nanotubes impart unique physical properties that make nanotubes suitable for a variety of applications. For instance, carbon nanotubes may exhibit electrical characteristics of metals or semiconductors, depending on the degree of chirality or twist of the tube. Different chiral forms of carbon nanotubes are referred to as armchair and zigzag, for example. The electronic properties exhibited by carbon nanotubes are determined in part by the diameter and length of the tube. Utilization of carbon nanotubes having particular electronic properties may be aided by a mechanism for producing or selecting nanotubes of specified dimensions.
Existing carbon nanotube synthesis techniques include arc discharge, gas phase synthesis, laser ablation of carbon, and the chemical vapor deposition of hydrocarbons. The arc discharge method is generally not able to control the diameter or length of carbon nanotubes. Meanwhile, the gas phase synthesis method, while appropriate for mass synthesis of carbon nanotubes, also has difficulty in controlling the diameter and length of the carbon nanotubes produced therefrom. While the laser ablation method, and the chemical vapor deposition method generally yield more uniform carbon nanotube products, no adequate mechanism exists for selectively synthesizing carbon nanotubes of specified dimensions.
Known selection and/or sorting techniques for carbon nanotubes having specified dimensions have also proven problematic. While an atomic force microscope or scanning tunneling microscope may be used to select and/or precisely measure the geometry of individual nanotubes, sorting large numbers of nanotubes via these instruments may be both time consuming and cumbersome. As applications for carbon nanotubes become more sophisticated, mechanisms for selectively manipulating carbon nanotubes of specified dimensions become an increasingly integral element of device construction.